Transmission And Substation Foundations - Technical Design Manual

SECTION 2: SOIL MECHANICS

Soil Mechanics

Cohesion When saturated clay is consolidated, that is, when the volume of voids decreases as a result of water being squeezed out of the pores, the shear strength increases with normal stress. If the shear strength of clays which have a previous history of consolidation (i.e., pre-consolidated) is measured, the relationship between shear strength and normal stress is no longer a line intersecting the ordinate at zero. The clays exhibit a memory, or cohesive shear strength. In other words, the clays remember the pre-consolidation pressure they were previously subjected to. This means considerable shear strength is retained by the soil. Figure 2-6 is an example of the relationship between shear strength and normal stress for a pre-consolidated plastic clay as derived from a triaxial shear test. The intersection of the line at the ordinate is called the cohesion.

Coulomb Equation for Shear Strength The equation for shear strength as a linear function of total stress is called the Coulomb equation because it was first proposed by Coulomb in 1773.

EQUATION 2-3

τ f = c + σtan

In terms of effective stress:

EQUATION 2-4

τ f = c’ + (σ - u) tan ’ where τ f = shear strength at failure c’ = cohesion

σ = total stress acting on the failure plane ’ = friction angle u = pore water pressure

angle of internal friction - ø

shear stress

Equations 2-3 and 2-4 are two of the most widely used equations in geotechnical engineering, since they approximately describe the shear strength of any soil under drained conditions. They are the basis for bearing capacity Equations 4-1 and 4-25 presented in Section 4.

cohesion

shear

shear strength strength

lower confining

normal stress normal stress

stress higher confining stress stress higher confining stress

maximum stress maximum vertical stress

Mohr’s Diagram for Moderately Plastic Soil Portland Cement Association (1996) Figure 2-6

Cohesion is analogous to two sheets of flypaper with their sticky sides in contact. Considerable force is required to slide one over the other, even though no normal stress is applied. Cohesion is the molecular bonding or attraction between soil particles. It is a function of clay mineralogy, moisture content, particle orientation (soil structure), and density. Cohesion is associated with fine grain materials such as clays and some silts.

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